Computerized Tomography (CT) scanners produce images of a subject by reconstruction of X-ray attenuation data acquired over multiple view angles. Typically, images are constructed by back projecting the view data received from the CT detector over the multiple views. CT images are representation of the X-ray attenuation coefficient at each image pixel, referred to as a CT number. The CT number typically provides information regarding density of the tissue. In medical imaging, a CT number −1000 is commonly associated with air (no attenuation) whereas CT number 0 is associated with water.
Analysis of image data can be qualitative or quantitative. In qualitative analysis, subjective viewing of images is used to identify boundaries of tissues, organs and foreign masses and/or to discriminate between tissue types based on differences in density across the image.
Quantitative analysis relies on the actual CT numbers of the image to provide information on actual densities and/or compositions of a Region Of Interest (ROI). Exemplary applications using qualitative analysis of CT images include Coronary Artery Calcium (CAC) testing to evaluate calcified plaque in the coronary arteries, bone mineral density evaluation, fat content of tissue evaluation, iron build up in liver evaluation, and contrast agent perfusion determination.
In order to perform quantitative analysis of true tissue densities, calibration of the CT numbers is performed to improve reliability of the CT numbers to be used to determine density.
Artifacts can be caused, for example, by beam hardening, thermal drifts in an acquisition system, inaccurate subtraction of a signal from scattered radiation, truncation as described herein below, and variability in tissue surrounding the field of view. Truncation artifacts typically occur when the scanned Field of View (FOV) is smaller than the patient size. Artifacts may impose a global shift in CT numbers such as global increase or global decrease of the CT number across the entire image or may impose spatial variations in CT numbers across an image so structures or tissues of uniform density display spatially varying CT numbers. In particular, artifacts known in the art as “cupping” involve CT number variation with radial symmetry relative to the image center.
CT scanners typically correct for spatial variations in CT number using a procedure known in the art as “phantom calibration.” Objects of known material and density are scanned prior to patient scan and calibration tables are calculated and applied to patient scan data. In addition, some CT scanners apply empirical correction known in the art as polychromatic correction, wherein scan data is arbitrary modified according to a formula or look-up table. Such corrections reduce the artifact level but cannot fully correct for every patient and every cross section within the patient.
Some calibration processes correct for relative efficiency and gain of the detector array elements and variation in X ray beam intensity across the irradiation field. One example of such calibration is known in the art as “air calibration”. Typically, air calibration involves performing a scan using a CT scanner, without there being a subject or phantom in the imaging space between the X ray source and the detector, so the detector array is irradiated by un-attenuated X ray beam. The acquired date, sometimes termed “air calibration data” is indicative of the relative efficiency and gain of the detector array elements and the variation in X ray beam intensity across the irradiation field. The air calibration data is used to normalize the attenuation data acquired during a subject scan.
U.S. Pat. No. 5,068,788 entitled “Quantitative computed tomography system,” the contents of which is incorporated by reference, describes a method of analyzing a suitably chosen CT number histogram to reduce the effects of background scatter and intermixing of tissues. CT numbers of individual tissues are obtained by locating leading edges of histogram distribution curves in regions of the histogram representing the individual tissues. The leading edge values are used as a starting point for construction of model curves representative of pure tissue samples against which the actual histogram distribution can be measured, by calculating and adjusting moments of the curve, following subtraction of assumed values for background and intermixing derived from the leading edge values. The adjusted CT numbers are used to create a reference plot by which other CT numbers can be converted to a physical quantity such as density for use in analyzing other tissues.
U.S. Pat. No. 6,990,222 entitled “Calibration of tissue densities in computerized tomography,” the contents of which is incorporated by reference, describes a hybrid calibration method that uses an calibration phantom (exterior reference) scanned simultaneously with a patient, and one or more known tissues of the subject (interior reference) to create a hybrid calibration reference that improves the measurement of tissue densities throughout the body. In addition, the calibration method is used to quantitatively define boundaries of tissue and organs for more accurate measurements of lengths, areas and volumes.
Some known image processing techniques for processing CT images involve forward projecting processed image data to reconstruct view data. After further processing, the forward projected data may be back projected to reconstruct the image data.
U.S. Pat. No. 5,008,822, entitled “Combined high speed back projection and forward projection processor for CT systems,” the contents of which is incorporated by reference, describes A CT scanner that generate views of data such as equal angular increment detector fan views which are convolved or filtered by an array processor. A combined back projector and forward projector back projects the data from the array processor into an output memory and forward projects lines of image representation data from an input memory to the output memory. Each view representation may again be an equal angular increment detector fan format view, a parallel ray format view, an equal linear increment source fan format view, a source fan format view, an equal angular incremental detector fan format, an equal linear increment detector fan format, or an equal angular increment source fan format.